Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are much less potent than estradiol, infants fed soy formula have serum concentrations that are up to 4 orders of magnitude higher than estradiol. We have been in the field with a longitudinal study of infants up to 9 months old, looking in detail at the growth and development of their estrogen-responsive tissue as well as sex hormones and gonadotropins. We have developed methods for studying play behavior, bone density, and language acquisition in 1 and 2 year olds, and completed a pilot recruitment evaluation. For all these studies, the exposed group are children using soy formula, compared with breast fed or cow milk formula.